This invention relates generally to a process for fabricating an integrated circuit, and more particularly to an improved method for fabricating metal interconnections in a semiconductor integrated circuit.
Aluminum and aluminum alloys are the most common materials used to fabricate metal contacts and interconnections in an integrated circuit built on a silicon substrate. These metals have several advantages over other conductive materials, most importantly, because of their low resistivity which results in an increase in device operating speed. In addition, these metals are relatively soft and flexible and, therefore, bond well to underlying layers and tend not to lift off. Aluminum is also a relatively inexpensive material.
However, the use of aluminum is not without its drawbacks. For example, aluminum spiking into the substrate may occur. Moreover, hillocking on the aluminum layer can be induced by thermal cycling during various processes, such as a high-temperature anneal step after the aluminum is deposited. This leads to reduced reliability and greater processing difficulties. In double level metal processes, for example, large hillocks can cause shorts between aluminum layers. Hillocks may also cause difficulties in photolithographic definition of fine lines due to irregular reflections.
It is known that the use of a refractory metal as the top layer of the metallization layer will reduce the likelihood of hillock formation on an aluminum layer. However, this can result in difficulties during final packaging, as refractory metals are brittle and are not easily processed at low temperatures, and therefore do not bond easily to gold or aluminum bonding wires. As a result, it is necessary to remove the upper refractory metal at the location of the bonding pads. This has, however, in the past required the performance of an additional photolithographic step or the formation of an additional metal layer of aluminum over the upper refractory metal layer.
It is an object of the present invention to provide a metallization process which allows contact to be made to the bonding pads during the fabrication of a semiconductor integrated circuit without the requirement of an additional photolithographic operation.
It is a further object of the present invention to provide an improved metallization process of the type described, which reduces significantly the formation of hillocks in the metal contacts and interconnections and substantially eliminates aluminum spiking.